Seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) of the strata underlying the seafloor. Among other things, seismic data acquisition involves the generation of acoustic waves and the collection of reflected/refracted versions of those acoustic waves to generate the image. This image does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing an improved image of the subsurface in a shorter period of time is an ongoing process in the field of seismic surveying.
Mapping subsurface geology during exploration for oil, gas, and other minerals and fluids uses a form of remote sensing to construct two-dimensional, three-dimensional or four-dimensional, i.e., time as the fourth dimension, images of the subsurface. The process is known as seismic surveying, wherein an energy source transmits pressure pulses into the earth. These pressure pulses can be reflected by geological interfaces associated with the earth and subsequently recorded at the surface by arrays of detectors. The arrays of detectors are configured as a receiver spread of a plurality of streamers towed in parallel and separated by tens to hundreds of meters with each streamer containing a plurality of receivers. The streamers can be configured to be towed in a flat, slanted or variable depth arrangement to optimize each receiver's recording capabilities for the associated bandwidth.
Conventional seismic surveys use one or more sources capable of generating frequencies of approximately 8 Hz to 70 Hz in bandwidth. FIG. 1 depicts a graph of these frequencies sampled in a conventional seismic survey. Although the conventional seismic data can be extrapolated to generate seismic images, producing a higher quality seismic image requires expanding the bandwidth of the seismic sources in both a lower 202 and a higher 204 frequency direction. In recent years advances such as a BroadSeis system by CGG Services SA have provided for a broadened bandwidth in both directions 202, 204 for seismic data collection as illustrated in the graph depicted in FIG. 2. More recent developments such as a BroadSource system by CGG Services SA have provided for extending the available sampling frequency at the high end 302 of the seismic sampling bandwidth as illustrated in the graph depicted in FIG. 3. Advances such as those described herein have increased the available sampling bandwidth to a range of from 6 Hz to 200 Hz.
In the above described seismic data acquisition systems, seismic data associated with the very low seismic bandwidth of 1 Hz to 6 Hz is generated based on extrapolation of the seismic data collected from the higher bandwidth. The calculated seismic data provides the theoretical data necessary to generate the seismic images but the images are of a lower quality because they are based on the theoretical very low seismic bandwidth data.
A next step in improving the quality of seismic imaging requires seismic sources capable of generating waves in the 1 Hz to 6 Hz bandwidth range, receivers capable of detecting waves in the 1 Hz to 6 Hz bandwidth and seismic survey configurations appropriate to include properly configured 1 Hz to 6 Hz bandwidth sources and receivers at locations appropriate for recording the 1 Hz to 6 Hz bandwidth waves.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks associated with seismic data acquisition at frequencies of 1 Hz to 6 Hz based on properly configuring low frequency sources and receivers.